Infra red (IR) seekers and radio frequency (RF) radars are well-known in the art to determine the position and velocity of distant objects.
For example, passive IR seekers can employ a forward looking infrared (FLIR) camera or other thermal imaging camera for detection of infrared radiation, typically emitted from a heat source, to create images of distant objects assembled for video output. IR seekers can be used to help pilots and drivers steer their vehicles at night, or to detect warm objects against a cooler background. Passive IR seekers offer accurate tracking capabilities due to their narrow beam width and high resolution of object details. However, IR seekers have limited operative capability in adverse weather conditions, when an object is obscured by clouds, fog, precipitation, smoke, dust or the like.
Active RF radar systems transmit radiation from antenna toward an object, which is then reflected back from the object towards the antenna. These systems can be employed in all weather. However, such RF systems provide limited discrimination between multiple objects, are limited in angular resolution, and are sensitive to radio frequency jamming.
It can be seen that the characteristics of IR seekers and RF radars are complementary. Therefore, dual mode electromagnetic detection systems are known that combine a passive infra red (IR) seeker and an active radio frequency (RF) radar in one device. These dual systems provide the advantages of both technologies.
In order to protect a thermal imaging camera of IR seekers and/or an antenna of RF radars from weather or conceal it from public view protective domes (also known as “radomes”) are used. A dome is a structural, weatherproof enclosure that is constructed of material that minimally attenuates the electromagnetic signals transmitted or received by the detection system.
Generally, a variety of materials have been used for constructing protective domes. Historically, balsa and plywood were used in early antenna radome structures. Modern ground-based and ship-based antenna radomes are manufactured using composite materials such as fiberglass, quartz, and aramid fibers held together with polyester, epoxy, and other resins.
When a detection system employs both IR camera and RF radar, there is an interest in using a single dome for the both devices. In this case, the dome must be transmissive to a selected portion of the RF spectrum and a selected portion of the IR spectrum.
Microwave transmission through a material is decreased due absorption and/or reflection. The microwave reflection and absorption can, for example, be characterized by frequency-dependent dielectric constant and loss tangent of the material. It is important that the dielectric constant of the material in the selected portion of the IR and RF spectra is low. A low dielectric constant material reduces reflections. Reduced reflections minimize impact to the radiation pattern and insertion loss.
U.S. Pat. No. 5,327,149 describes a dual-mode, radio frequency, optical-wavelength detector apparatus that includes RF-transparent optical elements adapted to focusing optical energy in a wavelength range including ultraviolet through infrared wavelengths, on an optical detector, and an RF antenna located behind the optical elements. The apparatus also includes a protective dome made of zinc sulfide (ZnS), which has a relatively low loss factor and suitable dielectric constant (i.e., tan δ=0.0024; ε=8.35) for transparency to both infrared and radio frequency energy.
U.S. Pat. No. 6,307,521 describes a reflector antenna arrangement that can transmit and receive both infrared (IR) and millimeter wave (RF) radiation. The arrangement includes a main reflector, a subreflector, an IR feed system, an RF feed system, a radome, and a bispectral window arranged in an opening provided in a central area of the main reflector. The radome has a hemispherical single layer having a constant wall thickness, and is made of a dielectric material, preferably ZnS.
U.S. Pat. No. 7,786,418 describes a multimode detector system including an imaging infrared (IIR) seeker and a radio frequency (RF) seeker for mounting on a missile. The IIR seeker and the RF seeker may view an outside scene through a dome. The dome may be transmissive to the selected portion of the RF spectrum and the selected portion of the IR spectrum. The dome may function to shield the components of the seeker from the outside environment and from the air stream when the missile is in flight. The dome may be essentially spherical, or may be conical, flat, or some other shape. The dome material may be glass, quartz, sapphire, or some other material selected for a combination of optical and mechanical properties.
U.S. Pat. No. 7,557,769 describes a radome and method making thereof. The radome comprises an electro-optic transparent substrate, a semiconductor coating on the substrate, and a frequency selective surface embedded in the coating. The substrate is one or a combination of glass, silicon, MgF2, ZnS, and spinels, most preferably sapphire (with a GaN coating). The coating is one or a combination of GaP, GaN, GaAs, SiC, and Si.